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How a Thermostat Works

Mechanical (top) and Electronic (bottom) Thermostats

Thermostats measure your home’s ambient temperature and use that information to activate your furnace or air conditioner, depending on the thermostat’s setting.

There are two main types of residential thermostats: programmable electronic and electromechanical.

Programmable Electronic Thermostats

Electronic thermostats utilize an electronic heat-sensing element and circuitry to sense temperature changes and turn on heating or cooling equipment. Like a small computer, they are programmable; their timers allow you to warm up your house before you get out of bed in the morning and before you come home from work and be set at different temperatures for different times of the day. This means you can closely align room temperatures with your needs, ensuring comfort without wasting energy.

Electomechanical Thermostats

Electomechanical thermostats have some type of mechanical temperature sensing device—typically a bi-metal coil or strip. As the name suggests, this type is made from two connected metals. Changes in temperature cause these metals to expand and contract at different rates, causing the coil or strip to move.[GARD align=”left”]

Electromechanical thermostat has a bi-metal coil that moves with temperature changes, changing the tilt on a small vial filled with mercury which completes an electrical circuit.

The bi-metal coil or strip is connected to a device that completes an electrical circuit. In the case of Honeywell’s electromechanical thermostats, a spiral bi-metal sensor attaches to a small glass vial that contains mercury; the coil’s movement causes the vial to tilt one way or the other.

Because one of the properties of mercury is that it conducts electricity, the circuit is completed when the mercury flows to one end of the vial where there are two separated electrical contacts.

A thermostat that operates both heating and cooling units has two contacts at each end of the vial. When the vial tilts in one direction, the mercury flows to that end and completes a circuit that calls for heat. When the system is switched to the cooling cycle, the mercury flows to the other end of the vial to turn on the cooling.

Disposal of the small amount of mercury from this type of thermostat has become an issue in recent years. Pilot programs are being tested in some states for recycling old mercury-containing thermostats; Honeywell has been a very active player in these initiatives.

Some electromechanical thermostats, such as those made by GE, work similarly but complete the circuit with a magnetic snap reed switch.

Zoned heating system utilizes multiple thermostats and dampers in the ductwork to regulate the delivery of heated and chilled air.

Zoned Heating & Cooling

Sophisticated zone-controlled home climate systems divide the house into several separate areas that may each be controlled by separate settings and times on individual thermostats. With zone controls, thermostats open and close dampers, sending warm or cool air when and where needed.[GARD align=”left”]

Find Pre-Screened Local Thermostat Installation Help

Electronic Thermostats Make Sense

Electronic thermostats offer considerably more control than electromechanical models, and their electronic sensing element is more responsive to temperature changes than bi-metal sensors.

Simplicity and readability are two important considerations when choosing an electronic thermostat.

A programmable electronic model allows you to set room temperature so that, in winter, your home’s temperature can dip down to a fairly low temperature—say, 60 degrees—after you’ve gone to bed and then rise to a comfortable 70 degrees when you get up in the morning. Or, if no one is home during the day, you can set it so the house cools down when you leave for work and returns to a comfortable temperature by the time you arrive home. Then, on the weekend, it can change to an entirely different schedule.[GARD align=”left”]

Using a programmable thermostat, you can eliminate considerable wasted energy. As a rule of thumb, every degree you turn down a thermostat will save 3 percent of energy costs over a 24-hour period. In other words, if you turn down the temperature from 70 to 61 degrees for an eight-hour period every night, you can save about 9 percent. Reducing daytime usage similarly can double your savings.

Matching Thermostat to Equipment

Beyond these two main differences, thermostats can differ by whether they are intended to control a furnace only, a furnaces and an air conditioner, a heat pump, or equipment that operates in multiple stages that kick-in as the need for heating or cooling increases.

Check the packaging for information about the kind of equipment a thermostat can handle.

Many thermostats have some type of adjustment that allows the device to suit the system: for example, they may have a small switch on the back or wires that are jumpered. This adjustment is necessary because some heating systems are much slower to heat up than others and cycle differently. And some types of heating require system fans and others don’t.

For example, most heat pumps have an auxiliary electric heating element that kicks on when the room’s temperature drops about 2 degrees lower than the thermostat’s setting (or “set point”). If the room temperature is allowed to drop to 60 degrees at night and is then turned up to 70 degrees in the morning, the auxiliary heat will come on until the room temperature reaches about 68 degrees. In most parts of the country, using this much electric heat is quite expensive.[GARD align=”left”]

A sophisticated electronic heat-pump thermostat automatically calculates when the heat must come on to bring a room’s temperature up to the set point by the time you’ve programmed. It tells the heat pump to bring the temperature up from 60 to 61 degrees, then from 61 to 62 degrees, and so forth. That way, the electric auxiliary heat is “fooled” into staying off.

If you have a system that provides multiple stages of heating and cooling, such as a dual-speed air conditioner, a furnace with two sets of burners, or a heat pump, you need a thermostat that is designed to handle this complication (some conventional thermostats have a jumper or switch that can be set to modify the thermostat for such equipment). Zoned heating systems that send warmth or cooling to various rooms of a home depending upon the needs of those rooms generally require special programmable electronic thermostats (you can really fine-tune your home’s areas for comfort with one of these).

Home Thermostats: from Simple to the Best

Unless you have a multiple-stage heating and/or cooling system that requires a very particular type of thermostat, choice usually boils down to personal preference, which is generally a dynamic of features and price.

State-of-the-art electronic thermostat “learns” your behaviors, eliminating the need to program it. Photo: Nest

People who are techno-phobic often prefer the simplicity of a conventional electromechanical model. Those who are interested in more control and potential energy savings opt for an electronic type. Because of its energy-saving advantages, we recommend electronic programmable thermostats.

Most electronic thermostats cost more than electromechanical types. For example, a professionally installed Honeywell T-87 Round electromechanical thermostat may run about $40. A high-end electronic model can cost up to about $200 installed (state-of-the-art models can cost more). You can, however, buy a very serviceable electronic thermostat for as little as $40.

Though all electronic thermostats may look alike, they’re not. If one is considerably cheaper than another, there is probably a reason for the price difference. Some low-priced ones may not be able to maintain a narrow “comfort window.” Others may not adequately anticipate at what point the furnace must turn off, thereby wasting residual heat and overshooting the desired room temperature. Equally important is the fact that some types are more difficult to read and program than others. This latter point is very important. If you can’t easily figure out how to use a programmable thermostat, you may not want to go to the trouble of programming it, negating any advantages and the expense and effort of installing it.

Today, most manufacturers are focusing on features. For example, in the pursuit of the ultimate user-friendly electronic thermostat, Carrier and Bryant have each developed a line of thermostats based on input from consumer focus groups. The resulting products are aesthetically pleasing, have large “positive action” push buttons made from soft rubber, and display useful feedback and system alerts such as a “clean filter” indicator. The units also automatically change over from heating to cooling. In the event of a power outage, they hold their programming up to 72 hours—without backup batteries.

Smartphone tied to state-of-the-art thermostat tracks home energy usage. Photo: Nest

These are the types of features to look for if you want a state-of-the-art thermostat that will offer you comfortable, efficient control for years to come.

Going several steps further is the Nest Learning Thermostat, developed by Silicone Valley techies in 2011. This sleek, simple device utilizes sensors, algorithms, cloud computing, and machine learning to learn the behaviors and preferences of a household, then controls the home’s temperature accordingly. It gathers data as you use it, eventually programming itself to meet your needs. And, because it hooks up to WiFi, it allows you to manage your home heating and cooling from a laptop, smartphone, or tablet. At a suggested retail of $249, it is at the top end of the price spectrum.

Home Thermostats

A home thermostat tells the furnace and/or cooling system when to turn on and off as temperatures in the house or individual rooms fluctuate. This is a very important job, because it affects both comfort and energy bills.

The first thermostat was invented in the early 17th century by a Dutch man, Cornelis Drebbel, who placed a float inside a mercury thermometer and connected that apparatus to a damper cover on a furnace. When the mercury climbed to a certain level, the float caused the damper to close. [GARD align=”left”]

Today’s thermostats do fundamentally the same thing, activating some type of switch with the rise and fall of room temperature, but the technology has come a long, long way since then. A state-of-the-art thermostat, probably best exemplified by the Nest Learning Thermostat, learns a family’s behaviors and preferences, digests this information, and controls the temperature accordingly—essentially programming itself. When connected to a home’s Wi-Fi, it can be managed remotely from a smartphone, tablet, or laptop computer. For more about this, see Home Thermostat Features.

Home Thermostat Buying Guide

Many homes built before the 1980s are equipped with obsolete, low-priced thermostats that did an acceptable job before home energy cost and usage became a major concern. Fundamentally, that older type of thermostat is okay as a simple heat-activated switch, but it lacks a level of automation and control that is necessary for energy efficiency. “Okay” isn’t good enough anymore. Older, low-efficiency thermostats waste energy and, because of this, they should be replaced with newer programmable electronic thermostats that can be programmed to maintain different temperatures during the day and night, eliminating periods when the furnace or air conditioning unit would otherwise run unnecessarily. For information on buying a new thermostat, see Thermostats Buying Guide.

How a Thermostat Works

Essentially a heat-activated switch, a thermostat has a temperature sensor that causes the switch to open or close, completing or interrupting an electrical circuit that runs the house’s heating or cooling system. It can do this job either mechanically or via electronic circuitry. For more about the technologies inside of thermostats, see How a Thermostat Works.[GARD align=”right”]

Thermostat Care & Repair

Because it is responsible for controlling the balance between your family’s comfort and the cost of home energy, a thermostat should be properly maintained. Although caring for a thermostat is normally quite easy, problems with the heating or cooling system can often be traced to thermostat issues. For information on thermostat care and repair, see Thermostat Repairs.

How to Replace a Thermostat

In most cases, replacing an older, non-programmable thermostat with a fully programmable model doesn’t require a professional HVAC contractor; you can do it with a little know-how. See How to Install an Electronic Thermostat for instructions on how to do this.

Featured Resource: Find Pre-Screened Local Thermostat Installation Help

How to Install an Electronic Thermostat

Programmable electronic thermostats help you save money and energy by allowing you to pre-program temperature settings for maximum efficiency during the day and night.

An electronic thermostat offers energy-efficient control.

These can easily replace single-setting thermostat models, as they also work with standard central air and heating systems.[GARD align=”left”]

Here is how to replace your older thermostat with a programmable model:

1Remove the old thermostat. After making sure the power to the furnace is disconnected, remove the old thermostat’s cover, unscrewing any screws that fasten it in place.

Disassemble old thermostat and label the wires according to their terminals.

2Carefully disconnect each wire from the thermostat, and bend them slightly so they do not sink behind the base. Place a small label at the end of each wire, writing down the letter that is next to each wire’s terminal.

3Unscrew and remove the base of the thermostat from the wall, and hold onto the wires as you do so. Secure the wires around a paper clip, pen, or other similar object that will weigh them down enough so that they do not slip backwards into the hole in the wall.

4Strip about half an inch of casing from the end of each wire if the wires are not already stripped. Rub the exposed copper with something made for cleaning metal, such as an emery cloth or a steel-wool sponge.

Thread the wires through the new thermostat’s base and attach it to the wall.

5Detach the base from the cover of  the new thermostat. Unfurl the pen or other object you have used as a weight from the wires, and loop them through the new base so that they appear under the terminal block. Reattach the weighted object to hold the wires in place.

6Mount the new thermostat’s base. Stuff a few paper towels into the hole in the wall behind the thermostat. (This will protect your device from drafts.) Place the base against the wall, and put a level on top to ensure it is evenly in place. Using a pencil, mark the areas where you will set your screws. Proceed to screw the base to the wall, using regular screws for wood and anchor screws for drywall.

7Connect the wires, following the instructions in your new thermostat’s manual and keeping wires separate from one another. If called for, install batteries into the device to provide a backup power supply. Attach the cover.[GARD align=”left”]

8Turn the power back on, and program your thermostat according to the manufacturer’s instructions.

Featured Resource: Find Pre-Screened Local Thermostat Installation Help

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